High density ethylene homopolymers and copolymers (HDPE) with higher olefins are widely used in injection molding operations. Advantageously such resins should have a narrow molecular weight distribution (MWD) which is largely determined by the nature of the catalyst. The catalyst should also exhibit other desirable characteristics for commercial use. For example, the productivity of the catalyst should be as high as possible so that the resin will have a high ratio of polymer to catalyst residue. It is also very desirable that the catalyst result in a polymer having a large particle size, an advantage which is particularly sought in gas-phase polymerization. Another desirable characteristic of the catalyst is that it have a hydrogen response. A high hydrogen response means that small increases in the amount of hydrogen used in the reactor (to control molecular weight in the known manner) will result in substantial decrease in molecular weight and a higher melt index polymer. The need for excessive amounts of hydrogen decreases the reactor volume available for the ethylene and other comonomers, thereby reducing productivity.
My U.S. Pat. No. 4,167,493 which is incorporated herein by reference discloses the treatment of magnesium oxide with methanol, a Lewis base, prior to impregnation with a titanium compound and an aluminum compound to form a Ziegler catalyst which produces high density polyethylene with a narrow MWD suitable for injection molding.
The production of narrow MWD high density polyethylene using a magnesium oxide supported catalyst is also disclosed in my U.S. Pat. No. 4,288,578 which is incorporated herein by reference. The catalyst in this patent is made by impregnating a magnesium oxide support with a mixture of titanium tetrachloride and tetrabutyltitanate and then an organoaluminum compound reducing agent.
In accordance with this invention a MgO supported catalyst is treated with an organic acid (a Lewis acid), then with the product of TiCl.sub.4 and a higher alkanol, and finally with an organoaluminum compound as a reducing agent. The catalyst is particularly suitable for preparing HDPE with narrow MWD and large particle size, excellent productivity and hydrogen response.